Fix C/C++ compiler generated warnings

 * Remove unused functions: -Wno-unused-function
 * Fix logical operations that are missing parentheses:
   -Wno-logical-not-parentheses, -Wno-parentheses
 * Add missing braces in initializers: -Wno-missing-braces
 * Remove unused variables: -Wno-unused-variable
 * Fix printf()-like formatting issues: -Wno-format
 * Avoid using unitialized variables:
   -Wno-maybe-uninitialized, -Wno-uninitialized
 * Fix a bug inside bta_gattc_get_gatt_db_impl()
   when the processed GATT DB attribute type is unknown.
 * Fix warnings about missing field initializers:
   -Wno-missing-field-initializers
 * Re-implement macro COMPILE_ASSERT(COND) to fix a compilation warning:
   -Wno-non-literal-null-conversion
 * Fix sign mismatch comparison warnings:
   -Wno-sign-compare
 * Fix warnings related to enum conversion:
   -Wno-enum-conversion
 * Fix warnings related to incompatible pointer types:
   -Wno-incompatible-pointer-types
 * Fix warnings related to enum conversion:
   -Wno-enum-conversion

Bug: 26879229
Change-Id: I522931fe156aeab23ae841051a9e25ceab00b1c2