libandroidfw: use map for more efficient indexOfString()

Bug: 249320624

There number of calls to ResStringPool::indexOfString() is very high
due to many calls to Resources.getIdentifier() in JAVA code. This is
used to get the resource ID (a 32-bit integer) by its resource name
(a string).

One example pprof trace is at:

  https://pprof.corp.google.com/?id=9b9b377bf52bdeb972c6ad154acf3902

This happens to the key string pool in a package, which is normally
encoded in UTF-8, unsorted. The call to indexOfString() ends up going
through the whole key string pool and do a per string comparison til
the string is found.

For some large applications, the key string pool could have > 10k+
strings, which results in an excessive calling to string8At() or
stringAt(). Although these calls are pretty quick, the excessive
callings are not necessary and inefficient.

There are a few ways to improve this:

1. sort the key strings so the current implementation ends up doing
a binary search. However, for some reason, there is no code in aapt
or aapt2 to support sorting and use the SORTED_FLAG.

2. or when needed, setup a std::map<StringPiece, size_t> to map the
key string to its ID, and use the map for all subsequent calls to
indexOfString()

Here we chose the 2nd way with a few justifications:

1. std::map<> has a stable O(logN) lookup performance, and a key
comparison doesn't involve comparing the whole key so it's both
fast and relative stable.

2. by using StringPiece, we are not allocating memory if the string
mapped doesn't need decoding, and StringPiece supports comparison.

This CL reduces the number of calls into string8At() or stringAt()
to the key string pool from 2m+ down to ~400k.

See example pprof trace at:

  https://pprof.corp.google.com/?id=cb677fca932260b2dc7db64ee37c4b61

Change-Id: I77a0ef13daac6d8a5c63a681c52f8ab9712578ed