Fix native memory leak caused by small HWUI path cache creation

HWUI calculates the texture size as w*h*bpp. In some cases, the
calculated path cache is small, but the actual memory allocated
in driver is 4k/8k/16k, much bigger than HWUI calculates.

Example: a 5*65 alpha texture, HWUI think it is 5*65*1 = 325 bytes,
but driver allocates 8K. An app can allocates up to 32M path textures,
which actually consumes 32M*(8*1024/325) = 806M memory.

Here we limit the number of path texture in the cache to 256, it
should be a pretty generous global limit.

Change-Id: I890819b73bb0b7f63e96bc3d9d0ff9469c16838c