Cherry-pick from master: AAPT2: Fix JavaDoc first sentence extraction.

The old algorithm for detecting the first sentence of a JavaDoc comment
looked for the first occurence of '.'. This does not work when code or a
{@link android.R.styleable} link is encountered in the first sentence.

Switch to checking for whitespace characters after the '.' character.

Bug: 62900335
Test: make aapt2_tests , make ds-docs
Original change-Id: I8238f6a6304c9c2f92e2e576ca8962a59c2b20ea

Change-Id: Ie272d0d793d157f2a30f3ead20b2b53976661d33