Reduce heap memory in minikin.

This patch reduces about 73 kB memory.
The original SparseBitSet could contain full 32bit integers, but all of
that is not necessary for Unicode code points. By reducing the supported
range to up to Unicode maximum, U+10FFFF, we can save extra memory.

SparseBitSet holds 256-bit sliced pages and indices of them.
Previously, we needed to hold up to 2^24-1 pages for keeping 32-bit
integers.

This CL limits the number of pages to 2^16-1 (65535), so that
SparseBitSet only supports 24-bit integers now, but this is sufficient
for keeping all Unicode code points. With this change, we can change the
index integer type from uint32_t to uint16_t.

Bug: 37357593
Test: minikin_tests passes
Change-Id: I462cc27927752c942ac5da0bf303a5afb81b87a3

• include/minikin/SparseBitSet.h[diff]
• libs/minikin/CmapCoverage.cpp[diff]
• libs/minikin/MinikinInternal.h[diff]
• libs/minikin/SparseBitSet.cpp[diff]
• tests/unittest/CmapCoverageTest.cpp[diff]