Fix mStringBlocks race in the AssetManager

There were a few places where access to the mStringBlocks were
not protected.

The crashes seen where similar to:

  java.lang.NullPointerException: Attempt to invoke virtual method \
    'java.lang.CharSequence android.content.res.StringBlock.get(int)' on a null object reference
  at android.content.res.AssetManager.getResourceValue(AssetManager.java:222)
  at android.content.res.ResourcesImpl.getValue(ResourcesImpl.java:188)
  at android.content.res.Resources.loadXmlResourceParser(Resources.java:2110)
  at android.content.res.Resources.getLayout(Resources.java:1111)

  java.lang.NullPointerException: Attempt to invoke virtual method \
    'java.lang.CharSequence android.content.res.StringBlock.get(int)' on a null object reference
  at android.content.res.AssetManager.getPooledStringForCookie(AssetManager.java:312)
  at android.content.res.TypedArray.loadStringValueAt(TypedArray.java:1212)
  at android.content.res.TypedArray.getValueAt(TypedArray.java:1198)
  at android.content.res.TypedArray.getColor(TypedArray.java:446)

What happened was that thread 1 was creating a new mStringBlocks in
makeStringBlocks while thread 2 was accessing mStringBlocks. The
makeStringBlocks starts off by overwriting mStringBlocks with a new
empty array and when thread 2 accessed its content NPE happened.

Bug: 30802713
Test: None (just added synchronization to help prevent races)
Change-Id: I810da26b161a6528b0dd241048dde5b239089244