Fix TextDropShadowCache infinite loop

Bug: 26862239

Switch TextDropCacheShadow to use the tracked objectSize()
instead of the optional bitmapSize. A mismatch here
results in ::get() infinite looping trying to free space in
the cache since the LRU removal callback would always
decrement mSize by 0 since bitmapSize was not being set.

Also prevent the infinite loop in the future by crashing if
this scenario happens again.

Change-Id: Ib4e9fbe1c8327af2335ad650fd694a1627d9824f