Android includes the Fraunofer FDK AAC Codec Library
Since Android 4.1, the Fraunhofer FDK library has been included in the Android open source project, the master code used for Android devices. AAC is also specified in the Android Compatibility Definition Document which lists the required features of Android devices.
Note that device manufacturers may supplement or replace the FDK with other software that meets the CDD requirements. A common example is when a DSP co-processor is used to offload music playback for lower power consumption. Thus, in some situations playback may differ from that of the FDK
The FDK is a quality fixed-point AAC implementation that also includes an encoder and supports low-delay varients of AAC such as AAC-LD and AAC-ELD for communication applications. AAC metadata and multichannel decoding is fully supported.
More information on the FDK is available on the main Fraunhofer site.
Android 4.1.1 had severe aliasing on 48 kHz content due to resampling. See this sweep resampling test. This was corrected in later releases.